I did this with an old vehicle taht was pretty basic without a lot of electronics.

the actual problem was sort of electronic-releated - leave lights on, drain battery to zero.

I put on a battery-cutout-monitor-thing. If battery drained to x%, it would disconnect the battery. to start the car, I think you had to stand on the brake pedal (add a load - like brake lights?) and it would reset and reconnect the battery - then you could start it.